This invention relates to liquid level measuring apparatus and more particularly, to apparatus for indicating the level of liquid in a high pressure boiler even while there is the continual conversion within the boiler of liquid to vapor as in a steam boiler.
In a boiler, high pressure and temperature and the presence of a condensate make accurate liquid level indication difficult. Steam boiler codes require the use of sight glass gauges, but the difference in the specific gravity of the water in the gauge and water in the boiler causes a significant variance in the level in the gauge and the level in the boiler. Very high pressures preclude the use of sight glass gauges.
Temperature-compensated Differential Pressure gauges are the primary device for automatic level indication. Gauges using a float are dependent on a buoyancy to weight relationship that at some point becomes adverse as specific gravity decreases and weight increases. Floats vented at the top for equilibrium sink when condensate collects in the float.
A variety of low pressure gauges have been designed using a float that either carries a magnet or is magnetic, to establish the location of the liquid level.
These devices use a technique of sensing whereby the float activates something like a switch or the float flips a magnetically responsive leaf. Some automatic devices use a nearby neon tube or set of magnetically responsive parallel wires positioned close to the line of travel of a magnet attached to the float. Some gauges use an exterior magnet or a magnetically responsive object positioned outside of the tank that follows the travel of a magnet attached to the float inside the tank. One such gauge uses a rolling follower while another uses a sphere in a column of liquid as a follower. Some gauges use multiple floats with a nearby reed switch and others use sequential switches on the exterior of the tank.
Each of these prior gauges has design limitations that make it inaccurate or non-functioning under conditions such as high temperatures and pressure.
Where floats have been used and the position of the float is transmitted outside the tank by a mechanical actuation (whether this is through magnetic coupling or otherwise), mechanically actuated gauges are often used to indicate the position of the float. As a rule, these gauges cannot be remotely located, but must be located on or close to the tank. In some cases, it is preferred to monitor a fluid level in the tank from a remote location and to present at the remote location a linear representation of liquid level in the tank which is capable of presenting an overall or coarse reading and a fine reading so that even very small fluctuations in the liquid can be monitored.
The prior techniques and devices described hereinabove represent but a few of the great many that have been proposed, tried and used in the past. These particular prior techniques are described herein, because the various novel features of the present invention avoid particular disadvantages of those prior techniques. Hence, those prior techniques are described in an effort to point out some of the advantages of certain features of the present invention.